Sorci Sess7 101309

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Sorci Sess7 101309

  1. 1. Impact of Levee Cutoff Walls on Groundwater Recharge Presented By: Paul Sorci, PE
  2. 4. Outline <ul><li>Sacramento River Location </li></ul><ul><li>Overview of How and Why Cutoff Walls Work </li></ul><ul><li>How Cutoff Walls are Constructed </li></ul><ul><li>How Cutoff Walls Affect Groundwater </li></ul><ul><li>Summary </li></ul><ul><li>Questions </li></ul>
  3. 5. Sacramento River Location Sacramento River Farmlands Downtown Sacramento Groundwater Analyzed
  4. 6. Overview of Cutoff Walls <ul><li>How They Work </li></ul><ul><ul><li>Creates a nearly impermeable vertical element that stops water from traveling horizontally through permeable layers </li></ul></ul><ul><ul><li>Lowers the amount of pore pressure at the landside levee toe due to high water levels </li></ul></ul>
  5. 7. Overview of Cutoff Walls <ul><li>How They Work </li></ul>Cutoff Wall Permeable Sands Fine Grained Soils Fine Grained Soils
  6. 8. Overview of Cutoff Walls <ul><li>How They Work </li></ul>
  7. 9. Overview of Cutoff Walls <ul><li>Multiple Methods of Installation in Natomas </li></ul><ul><ul><li>Deep Soil Mixing </li></ul></ul><ul><ul><li>Slurry Trench Method </li></ul></ul>
  8. 10. Cutoff Wall Construction <ul><li>Deep Soil Mix Method </li></ul>Permeable Sands Low Permeable Clay/Silt Levee/Blanket Layer Ground Surface 5’ Min. 6’ Wide Element 2’ Overlap
  9. 11. Cutoff Wall Construction <ul><li>Deep Soil Mix Method </li></ul>Previously Mixed Element Next Element Alignment Next Element
  10. 12. Cutoff Wall Construction <ul><li>Deep Soil Mix Method </li></ul>Slurry Injection at Auger Tips
  11. 13. Cutoff Wall Construction <ul><li>Slurry Trench Method </li></ul>Permeable Sands Low Permeable Clay/Silt Levee/Blanket Layer Ground Surface
  12. 14. Cutoff Wall Construction <ul><li>Slurry Trench Method </li></ul>Spoils Used for Mixing Consisting Mostly of Clay Excavated Trench Bentonite Slurry
  13. 15. Cutoff Wall Construction <ul><li>Slurry Trench Method </li></ul>Mixing Excavator Bentonite Slurry Mix of Soil, Cement, and Bentonite
  14. 16. Cutoff Wall Construction <ul><li>Slurry Trench Method </li></ul>Trench Backfill Trench Mix
  15. 17. Cutoff Wall Construction <ul><li>Compare and Contrast </li></ul><ul><ul><li>DSM </li></ul></ul><ul><ul><ul><li>Mixes in situ soils – less spoils </li></ul></ul></ul><ul><ul><ul><li>Smaller construction footprint – less equipment </li></ul></ul></ul><ul><ul><li>Slurry Trench </li></ul></ul><ul><ul><ul><li>Allows engineer to see material wall is keying in </li></ul></ul></ul><ul><ul><ul><li>Better quality control </li></ul></ul></ul>
  16. 18. Groundwater Affects <ul><li>Problem </li></ul><ul><ul><li>Soil bentonite (cutoff) walls will reduce groundwater recharge to landside of levee </li></ul></ul><ul><ul><li>Perform seepage analysis to evaluate </li></ul></ul><ul><ul><ul><li>Estimate groundwater flow with and without cutoff wall in place </li></ul></ul></ul><ul><ul><ul><li>Estimate effect of nearby canals on groundwater recharge </li></ul></ul></ul>
  17. 19. Groundwater Affects <ul><li>Two Types of Anlaysis </li></ul><ul><ul><li>Steady State </li></ul></ul><ul><ul><li>Transient </li></ul></ul><ul><ul><li>Did not account for 3-D effects i.e. flow around edges of cutoff wall </li></ul></ul><ul><ul><li>Use hydrograph to perform transient analysis </li></ul></ul>
  18. 20. Groundwater Affects <ul><li>Parameters Selected </li></ul><ul><ul><li>Permeability Modeled as 10 -6 cm/sec </li></ul></ul><ul><ul><li>Stratigraphy Different at Sections </li></ul></ul><ul><ul><ul><li>Wall extends 5 feet into fine grained foundation </li></ul></ul></ul><ul><ul><ul><li>Material beneath wall either sand or clay depending on stratigraphy </li></ul></ul></ul><ul><ul><ul><li>Cutoff wall less effective when sand beneath wall </li></ul></ul></ul>
  19. 21. Groundwater Affects <ul><li>Steady State Seepage Analysis </li></ul><ul><ul><li>Stations 70+00 cutoff wall fully penetrates sand layer </li></ul></ul><ul><ul><li>Station 353+00 cutoff wall only partially penetrates sand layer </li></ul></ul><ul><ul><li>Aquifer only recharges when river level above groundwater level </li></ul></ul>
  20. 22. Groundwater Affects <ul><li>Steady State Seepage Analysis (cont) </li></ul><ul><ul><li>Cutoff wall reduces seepage quantities up to 85 percent when sand layer is fully penetrated (Station 70+00) </li></ul></ul><ul><ul><li>Cutoff wall reduces seepage quantities up to 40 percent when sand layer is partially penetrated (Station 353+00) </li></ul></ul>
  21. 23. Station 70+00 85% Reduction Station 353+00 40% Reduction Cutoff Wall Tip in Fine Grained Soils Cutoff Wall Tip in Coarse Grained Soils
  22. 24. Groundwater Affects <ul><li>Transient Seepage </li></ul><ul><ul><li>Verify and validate steady state seepage </li></ul></ul><ul><ul><li>Hydrograph used to create transient time steps </li></ul></ul>Rainy Season – Groundwater Recharge
  23. 25. Groundwater Affects <ul><li>Transient Seepage </li></ul><ul><ul><li>Analysis performed where sand layer is fully penetrated (Station 70+00) </li></ul></ul><ul><ul><ul><li>Highest reduction in groundwater seepage </li></ul></ul></ul><ul><ul><li>Cutoff wall reduces seepage quantities up to 70 percent, as compared to 85 percent for steady state seepage </li></ul></ul><ul><ul><li>Steady state seepage can conservatively approximate the effect of the cutoff wall </li></ul></ul>
  24. 26. Groundwater Affects <ul><ul><li>Overall, the cutoff wall will reduce groundwater aquifer recharge by about 20-25 percent </li></ul></ul><ul><ul><li>Effect may be lower due to 3-D flow </li></ul></ul>Difference in Groundwater Recharge Rainy Season
  25. 27. Groundwater Affects <ul><li>Canals </li></ul><ul><ul><li>Canal modeled about 800 feet from landside toe of levee </li></ul></ul><ul><ul><li>Time dependent boundary condition </li></ul></ul><ul><ul><li>Canal contained 5 feet of water from May to November </li></ul></ul><ul><ul><li>Canal empty December to April </li></ul></ul>
  26. 28. Groundwater Affects <ul><li>Canals </li></ul><ul><ul><li>Minimal impact of canal on groundwater elevations </li></ul></ul><ul><ul><li>Groundwater around cutoff wall increases in summer months </li></ul></ul><ul><ul><ul><li>Due to water flow in canal and decrease of recharge to aquifer from the cutoff wall </li></ul></ul></ul>
  27. 29. Summary <ul><li>Cutoff Walls </li></ul><ul><ul><li>Creates a nearly impermeable vertical element that stops water from traveling horizontally through permeable layers </li></ul></ul><ul><ul><li>Have been installed using a Deep Soil Mixing or Slurry Trench Method in Natomas Basin </li></ul></ul>
  28. 30. Summary <ul><li>Groundwater Affects </li></ul><ul><ul><li>Steady state conservatively estimates recharge quantities </li></ul></ul><ul><ul><li>Locally, areas may see reduction in recharge up to 85 percent </li></ul></ul><ul><ul><li>Overall, recharge reduced 20 to 25 percent </li></ul></ul><ul><ul><li>Cutoff wall could impede seepage to river during summer months </li></ul></ul><ul><ul><li>Canal and cutoff wall may locally increase groundwater levels in summer months </li></ul></ul>
  29. 31. Questions ?

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